The present invention relates to a method for manufacturing a reinforcing fabric for a pneumatic tire. More particularly, the present invention relates to a method for constructing the body ply of a radial tire.
The body ply of a radial tire is generally composed of equally spaced cords or cables, which are disposed at substantially 90.degree. to the bead wires of the tire, and which are encased in an elastomeric material, generally rubber. Tire cords or cables are generally composed of a material such as rayon, nylon, polyester, fiber glass or steel and the individual tire cords or cables are generally constructed by twisting, stranding or bundling a plurality of fine filaments of one of these materials. For example, the number of individual filaments used to make up a single tire cord could range from 5 to 50 filaments.
In order to obtain the proper strength characteristics for a given size tire which is expected to accept a given load condition, a selection of cord material is initially made, the diameter, number of filaments, twist etc. of the individual cords are determined, and the spacing between the individual cords (ends per inch) is varied to adjust the ultimate strength of the completed body ply. Increased strength requirements for the body ply may further be provided by constructing the body ply of a plurality of superimposed layers of the reinforcing fabric, i.e., the sheet of rubber enclosing the plurality of space tire cords.
The process presently used to produce radial tire body plies, and for that matter reinforcing fabric for body plies and/or belts for radial as well as bias ply tires, consists of the use of a so-called "calender line". In this process, compounded uncured rubber is conveyed to a three or four roll calender which works the uncured rubber into a sheet or film form. The individual tire cords are set side by side, ranging from 10 to 25 ends per inch, and are fed to the rollers of the calender which then presses the sheet or film of uncured rubber onto the tire cords. Generally a calender of this type is 60 to 140 inches in width, and uses temperature controlled steel rolls ranging from 40 to 100 inches in diameter.
The individual tire cords are fed into the calender rolls from a creel room having hundreds of spools of individual tire cords. The spacing and tension of the cords are controlled by highly technical mechanical means, and the rubber or sheet form thickness is controlled by precision adjustment of the nip or space between the calender rolls.
The calendering operation produces properly spaced tire cords surrounded by uncured rubber in a continuous sheet with the tire cord extending along the length of the sheet and with the sheet having a thickness ranging from about 0.03 to 0.15 inches. The resulting sheet of calendered tire cord, i.e., the reinforcing fabric, is then formed into a large roll, usually after applying a plastic protective backing film in order to allow for subsequent unrolling. The reinforcing fabric is later unrolled and cut into sections or pieces of appropriate width and length depending on the size and type of tire to be constructed. For example, for radial tires, the sections to be utilized for the body ply are cut at 90.degree. to the length of the sheet.
As can easily be appreciated, the equipment required to accomplish the above calendering process is massive, costly, and requires considerable skill on the part of the calendering and cutting operators to produce uniform and acceptable body plies. Additionally, the calendering process presents difficulties when making any change from one type of tire cord to another or when changing the end count to accomplish a different strength rating of the finished tire. Moreover, the present practices, including the calendering operation, for making body plies are essentially limited to the use of pretwisted or bundled tire cords, i.e., the calendering process is not generally applicable to the use of individual filaments, for example, individual steel filaments, as the reinforcing element of the tire fabric since, in view of the greatly increased number of filaments required in order to provide the desired strength, the size and cost of the calendering line would be increased many fold. The use of twisted or cabled tire cord, however, as opposed to individual filaments, substantially increases the cost of material for the tire since a pretwisted tire cord costs nearly twice that of the untwisted filaments contained in the same cord.
Finally, another problem area with the present commonly employed practice of making body plies for radial tires is the splicing together of the ends of the section of reinforcing fabric on a tire building drum prior to the application of the bead wires. Due to the fact that the cords are placed side by side, it is often necessary to overlap the cords during the splicing operation, resulting in a narrow area of increased thickness which is undesirable. Alternatively, when butt splices are employed, a weakness is often created in the body ply which permits the cords to separate beyond acceptable standards when the tire is expanded into the final polaroid position.
In order to avoid the need for calendering when producing the reinforcing fabric, and to permit the use of individual filaments, for example, steel filaments, it is known in the art to form the reinforcing fabric from an extruded ribbon of uncured elastomeric material, for example, rubber, having steel filaments embedded therein. For example, see U.S. Pat. No. 3,778,329 issued Dec. 11th, 1973 to S. W. Alderfer. According to the teachings of this patent, which is directed to a belted/bias-ply tire but additionally teaches that belts for radial tires may be similarly manufactured, the extruded ribbon is placed on the surface of the drum so that successive wraps of the ribbon are contiguously wound so as to produce an annulus of predetermined size. The annulus is then helically slit to form the desired ply. While the teachings of this patent would appear to overcome some of the problems involved with a calendering process, this patent is not directed to the manufacture of radial tires, and sheets of reinforcing fabric formed according to the teachings of this patent would not be applicable for the body ply of a radial tire. Additionally, the teachings of this patent do not overcome many of the problems mentioned above relative to radial tires, and moreover, according to the specific teachings of this patent, the individual steel filaments must delineate a cylindrically helical path through the the ribbon, and thus through the fabric, producing additional problems during the manufacturing process.